YAC, a Moloney virus induced tumor of A/J mice, is a completely ineffective inducer of immunological responses. A/J mice injected with such inactivated tumors do not generate humoral or cellular cytotoxic responses and in addition do not reject viable challenge of homologous neoplastic cells. This tumor grows very fast in the syngeneic host, and even as low a dose as ten cells kills a mouse in seven weeks. The ability of YAC to grow in an uncontrolled manner in its syngeneic host, is probably linked to its capacity to stimulate the appearance of suppressor cells. In vitro cultivation of YAC tumor for a short period of time changes completely the immunological properties of the tumor. The in vitro cultivated tumor now designated YAC-1 induced cellular cytotoxic responses in the syngeneic host and in addition A/J mice injected with this inactivated tumor reject viable challenge of homologous neoplastic cells. Thus this tumor induces anti-tumor reactive cells rather than suppressor cells. Viable YAC-1 tumor grows slowly in the syngeneic host and a dose of 104 cells is required to kill a mouse in seven weeks. The purpose of our next studies is to define the differences between YAC and YAC-1 on a molecular level. We shall try to separate the various subcellular fractions of both tumors and to isolate from each fraction immunogenic determinants with defined molecular weights. In addition, we shall try to prove our hypothesis that YAC tumor contains both immunogenic determinants and suppressogenic determinants; and that amputation of the suppressogenic determinants that induce suppressor cells, will permit the expression of the immunogenic determinants. Our preliminary results agree with these basic assumptions.